alt Hacker News> Your player scans your library and indexes/sorts it however you'd like.
Which means the concepts of files and folders becomes immaterial. If a music player is only interfacing with a database of music metadata it doesn't matter how the bytes on disk are organized.
There's a reason there's been 30+ years of file systems trying to tack on database functionality (BeFS, WinFS, etc) or over the top metadata indexing (Spotlight, Lucene, etc) to file systems. The files and folders abstraction is not sufficient for non-technical users in many cases.
The music player isn't the only thing interfacing with those files. Backups do. Network file shares do. Pendrives, dedicated music players, and children's toys that can load music do. The player changes over the years (winamp, VLC, amarok, clementine, jellyfin, etc.). Sometimes "apps" go wonky and the only way to fix them is to "clear user data", which is opaque, and you'd rather control that/be more selective.
I don't really care how the player stores its metadata (the secondary indexes it makes after the scan), but I don't want it mucking with the original data, which I've had and will continue to have far longer than the player. Files in a known format (e.g. .flac) offer a standard, common interface for storing that data that's easy to organize and exchange across computers.
Google drive also shows what a world where people don't use folders looks like: I can never find documents at work without a link even when I have an idea of what I'm trying to search for. Folders at least push the user to do some organization. GDrive happily encourages you to do none.